1. Field of the Invention
This invention relates to apparatus incorporating solid state inverters for providing compensation on electric power lines. More particularly, it relates to a dynamic voltage restorer which injects a compensating voltage in series with the power line to offset voltage sags, such as may be caused by an upstream fault. The compensating voltage is injected using the primary winding or a delta connected secondary winding of a load transformer, which in many cases, is already provided for voltage transformation.
2. Background Information
In existing dynamic voltage restorer (DVR) designs, a partially rated compensating voltage source is injected in series into a utility power line. This requires isolation transformers between the voltage source inverter that generates the compensating voltage and the feeder where it is injected. These injection transformers, which must be provided for each phase, have to be rated to handle two per unit or more flux excursions to enable a range of wave shapes to be synthesized. Thus, these transformers are large and costly.
To avoid the need for injection transformers and their associated problems, U.S. patent application Ser. No. 09/017,034, filed on Feb. 2, 1998, describes an arrangement for placing the inverter at the neutral of the secondary winding of a load transformer. Three or four wire wye type systems can be handled using this technique. One drawback of this arrangement is that the inverter is placed on the low-voltage side of the transformer. The DVR inverter must therefore handle high currents at relatively low voltage. The forward-conduction voltage drop of the inverter switches will be a significant portion of the total ac voltage, resulting in relatively high losses. No arrangement is described for injection into a delta-connected load transformer.
Thus, there is a need for an improved DVR which eliminates, or at least reduces, the requirement for injection transformers to insert the compensating voltages into the power line.
There is a particularly need for such a DVR with improved efficiency.
There is also a need for such a DVR which can be used with power lines with any number of phases.
There is especially a need for such a DVR which can be used with three-phase systems with either delta or wye connected transformer windings.
There is a further need for such an improved DVR which can use a load transformer to inject the compensating voltage into the power line, especially in systems where such a transformer is already available.